Superconducting Quantum Interference Devices (SQUIDs) are often used as highly sensitive magnetic field sensors. Such SQUID sensors are becoming increasingly popular due to the capabilities of high sensitivity sensing in areas such as geophysical mineral prospecting and biological magnetic field detection, such as magnetic field emanations from the human brain or other human organs.
With the advent of high critical temperature superconducting (HTS) materials such as YBa2Cu3Ox (YBCO), HTS-SQUIDs can operate at or above 77K (−196° C.) and hence can be cooled by relatively inexpensive liquid nitrogen, rather than requiring liquid helium as a coolant for operation at 4K (−269° C.). Liquid nitrogen is also more convenient to use than liquid helium, allowing the system as a whole to be made in a compact form.
The use of high-temperature superconducting (HTS) materials for the fabrication of SQUID based magnetometers and gradiometers is now fairly well established (For example, W. Eldelloth, B. Oh, R P. Robertazzi. W. J. Gallagher, R. H. Koch, Appl. Phys. Lett., 59, 3473 (1991); S. Knappe, D. Drung, T. Schurig, H. Koch, M. Klinger, J. Hinker, Cryogenics. 32, 881, (1992); M. N. Keene, S. W. Goodyear, N. G. Chew, R. G. Humphreys, J. S. Satchell, J. A. Edwards, K. Lander, Appl. Phys. Lett. 64, 366 (1994); G. M. Daalmans, Appl. Supercond. 3, 399, (1995); M. I. Faley, U. Poppe, K. Urban, H.-J. Krause, H. Soltner, R. Hohmann, D. Lomparski, R. Kutzner, R. Wordenweber, H. Bousack, A. I. Braginski, V. Y. Slobodchikov, A. V. Gapelyuk, V. V. Khanin, Y. V. Maslennikov, IEEE Trans. Appl. Supercond., 7, 3702 (1997)). Despite the significant advantages which accrue from being able to operate at liquid nitrogen temperatures, HTS materials remain more difficult to use than the alternative low-temperature superconducting materials, and many design practices in low temperature helium cooled superconductors (LTS) cannot be implemented in HTS materials. In particular, the lack of HTS superconducting wires and the difficulty of forming superconducting connections in HTS materials means that the standard LTS design practice of forming gradiometer coils from superconducting wires, is not applicable in HTS materials.
Designs for HTS gradiometers sensitive to the on-diagonal components, ∂Bi/∂Xi (axial gradiometers), have been described (for example: R. H. Koch, J. R. Rozen, J. Z. Sun, W. J. Gallagher, Appl. Phys. Left., 63, 403,(1993); H. J. M. ter Brake, N. Janssen, J. Flokstra, D. Veldehuis, H. Rogalla, IEEE Trans. Appl. Supercond., 7, 2545, (1997); J. Borgmann, P. David, G. Ockenfuss, R. Otto, J. Schubert, W. Zander, A. J. Braginski, Rev. Sci. instrum. 68, 2730,(1997) but these have been implemented only by means of electronic or software subtraction of the outputs of a pair of SQUID magnetometers which are generally positioned at fixed distances from each other on a common normal axis. These designs suffer from the disadvantage that both magnetometers must operate linearly in the full ambient field (often the earth's magnetic field). It is difficult to achieve good common-mode rejection (rejection of homogeneous fields) which is generally limited to an order of about 10−3 in most implementations, Furthermore, the achievable noise performance can be dependent upon the magnitude of the background homogeneous field; being determined by microphonics which arise from vibrations causing randomly varying misalignment of the axes of symmetry of the two SQUIDs.
Some of these problems are ameliorated by the use of intrinsic gradiometer structures. Although several designs for intrinsic magnetic gradiometers utilising HTS films have been described in the literature these designs are sensitive only to the off-diagonal components of the first-order gradient tensor, ∂Bi/∂Xj, i≠j (transverse gradiometers). These designs generally fall into one of two types. The first employs a “figure eight” topology in which the gradiometric pick-up loop structure consists of a pair of superconducting loops with a common conductor that is interrupted by a direct current (DC) SQUID. The SQUID operates as a two-port device (SQUID amplifier) because the flux in the SQUID is derived from the current directly injected into a pair of input terminals. Depending upon the matching of the inductances and equivalent magnetic areas of the gradiometer input loops the current in the SQUID is proportional to the difference in the shielding currents induced in the pick-up loops in response to an external magnetic field gradient. The two pick-up loops are electrically in parallel, so one disadvantage of this topology is that even in a homogeneous field a large overall shielding current is induced in the outer perimeter of the pick-up loop structure with the potential to degrade noise performance through the associated production of large numbers of Abrikosov vortices in the superconducting film.
Another approach to the development of HTS transverse gradiometers employs a planar pick-up loop structure that is flip-chipped with a SQUID magnetometer to which it is inductively coupled. In the first-order designs the flux transformer consists of a pair of pick-up loops, one of which is coupled to the SQUID magnetometer. By matching the mutual inductance between the SQUID and the loop, the total effective magnetic area of the SQUID/loop combination can be made exactly opposite to that of the other loop of the flux transformer. Under these conditions the sensitivity to a homogeneous magnetic field vanishes but remains non-zero with respect to a magnetic field gradient. A second-order transverse gradiometer has also been implemented using this approach.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Throughout this specification, the terms ‘superconducting material’, ‘superconducting device’ and the like are used to refer to a material or device which, in a certain state and at a certain temperature, is capable of exhibiting superconductivity. The use of such terms does not imply that the material or device exhibits superconductivity in all states or at all temperatures.